Archive for December 2006

As I don’t expect to be able to blog again before the Xmas break, I’d like to wish all readers of QQ a happy whateveryou’recelebrating.

The holidays are one of those times of year when relatives get the opportunity to ask you, “So, what exactly is it that you do research on?”. This dreaded question will come with certainty, regardless of how many times you have previously explained it to them. It’s not their fault because the average person does not have physics on their mind for any significant amount of time, so it’s easy to forget what it’s all about.

The question is especially bad if you spend any time thinking about the foundations of quantum theory, because it’s difficult to describe quantum theory accurately in a few words. Here’s my best shot at an answer at the moment.

Miscellaneous Relative: So, what is this quantum theory thing all about then?

Me: Well, it’s not exactly about the fact that particles sometimes behave like waves and waves like particles.

MR: Go on.

Me: There is this thing called the Heisenberg uncertainty relation, but strictly speaking it doesn’t say that a measurement of position necessarily disturbs the momentum and vice-versa.

MR: OK.

Me: And it’s definitely not that there are multiple universes.

MR: That’s a shame. I enjoy science fiction, so that was the bit I liked the most.

Me: There are these things called wavefunctions, which can be in superpositions, but it’s not entirely clear what the true significance of that is.

MR: I’m not getting much insight into what you actually do from this by the way.

Me: It seems that John Bell proved that locality and realism are incompatible, but people are still debating the significance of that, so it’s definitely not the whole story either.

MR: Now I really have no clue what you are talking about.

Me: It’s not just about “finding the right language” with which to talk about physics. In particular, I don’t think that revising logic is really the right thing to do.

MR: That sounds sensible enough.

Me: Some people think the whole thing is just about doing something called “solving the measurement problem”, but I don’t think that’s an entirely helpful way of looking at things.

Firstly, he does believe that the whole universe obeys the laws of quantum mechanics, which are not required to be generalized.

Secondly, he does not think that Everett/Many-Worlds is a good way to go because it doesn’t give a well-defined rule for when we see one particular outcome of a measurement in one particular basis.

He believes that collapse is a real phenomenon and so the problem is to come up with a rule for assigning a basis in which the wavefunction collapses, as well as, roughly speaking, a spacetime location at which it occurs.

For now, he describes collapse as an unanalysed fundamenally stochastic process that achieves this, but he recognizes that it might be useful to come up with a more detailed mechanism by which this occurs.

Steane’s problem therefore reduces to picking a basis and a spacetime location. For the former, he uses the standard ideas from decoherence theory, i.e. the basis in which collapse occurs is the basis in which the reduced state of the system is diagonal. However, the location of collapse is what is really interesting about the proposal, and makes it more interesting and more bizzare than most of the proposals I have seen so far.

Firstly, note that the process of collapse destroys the phase information between the system and the environment. Therefore, if the environmental degrees of freedom could ever be gathered together and re-interacted with the system, then QM would predict interference effects that would not be present if a genuine collapse had occurred. Since Steane believes in the universal validity of QM, he has to come up with a way of having a genuine collapse without getting into a contradiction with this possibility.

His first innovation is to assert that the collapse need not be associated to an exactly precise location in spacetime. Instead, it can be a function of what is going on in a larger region of spacetime. Presumably, for events that we would normally regard as “classical” this region is supposed to be rather small, but for coherent evolutions it could be quite large.

The rule is easiest to state for special cases, so for now we will assume that we are talking about particles with a discrete quantum degree of freedom, e.g. spin, but that the position and momentum can be treated classically. Now, suppose we have 3 qubits and that they are in the state |000> + e^i phi |111>. The state of the first two qubits is a density operator, diagonal in the basis {|00>, |11>}, with a probability 1/2 for each of the two states. The phase e^i phi will only ever be detectable if the third qubit re-interacts with the first two. Whether or not this can happen is determined by the relative locations of the qubits, since the interaction Hamiltonias in nature are local. Since we are treating position and momentum classically at the moment, there is a matter of fact about whether this will occur and Steane’s rule is simple: if the qubits re-interact in the future then there is no collapse, but if they don’t then the then the first two qubits have collapsed into the state |00> or the state |11> with probability 1/2 for each one.

Things are going to get more complicated if we quantize the position and momentum, or indeed if we move to quantum field theory, since then we don’t have definite particle trajectories to work with. It is not entirely clear to me whether Steane’s proposal can be made to work in the general case, and he does admit that further technical work is needed. However, he still asserts that whether or not a system has collapsed at a given point is spacetime is in principle a function of its entire future, i.e. whether or not it will eventually re-interact with the environment it has decohered with respect to.

At this point, I want to highlight a bizzare physical prediction that can be made if you believe Steane’s point of view. Really, it is metaphysics, since the experiment is not at all practical. For starters, the fact that I experience myself being in a definite state rather than a superposition means that there are environmental degrees of freedom that I have interacted with in the past that have decohered me into a particular basis. We can in principle imagine an omnipotent “Maxwell’s demon” type character, who can collect up every degree of freedom I have ever interacted with, bring it all together and reverse the evolution, eliminating me in the process. Whilst this is impractical, there is nothing in principle to stop it happening if we believe that QM applies to the entire universe. However, according to Steane, the very fact that I have a definite experience means that we can predict with certainty that no such interaction happens in the future. If it did, there would be no basis for my definite experience at the moment.

Contrast this with a many-worlds account a la David Wallace. There, the entire global wavefunction still exists, and the fact that I experience the world in a particular basis is due to the fact that only certain special bases, the ones in which decoherence occurs, are capable of supporting systems complex enough to achieve conciousness. There is nothing in this view to rule out the Maxwell’s demon conclusively, although we may note that he is very unlikely to be generated by a natural process due to the second law of thermodynamics.

Therefore, there is something comforting about Steane’s proposal. If true, my very existence can be used to infer that I will never be wiped out by a Maxwell’s demon. All we need to do to test the theory is to try and wipe out a conscious being by constructing such a demon, which is obviously impractical and also unethical. Needless to say, there is something troubling about drawing such a strong metaphysical conclusion from quantum theory, which is why I still prefer the many-worlds account over Steane’s proposal at the moment. (That’s not to say that I agree with the former either though.)

It appears that I haven’t had a good rant on this blog for some time, but I have been stimulated into doing so by some of the discussion following the Quantum Pontiff‘s recent post about Bohmian Mechanics. I don’t want to talk about Bohm theory in particular, but to answer the following general question:

Just what is the goal of studying the foundations of quantum mechanics?

Before answering this question, note that its answer depends on whether you are approaching it as a physicist, mathematician, philosopher, or religious crank trying to seek justification for your outlandish worldview. I’m approaching the question as a physicist and to a lesser extent as a mathematician, but philosophers may have legitimate alternative answers. Since the current increase of interest in foundations is primarily amongst physicists and mathematicians, this seems like a natural viewpoint to take.

Let me begin by stating some common answers to the question:

1. To provide an interpretation of quantum theory, consistent with all its possible predictions, but free of the conceptual problems associated with orthodox and Copenhagen interpretations.

2. To discover a successor to quantum theory, consistent with the empirical facts known to date, but making new predictions in untested regimes as well as resolving the conceptual difficulties.

Now, let me give my proposed answer:

To provide a clear path for the future development of physics, and possibly to take a few steps along that path.

To me, this statement applies to the study of the foundations of any physical theory, not just quantum mechanics, and the success of the strategy has been born out in practice. For example, consider thermodynamics. The earliest complete statements of the principles of thermodynamics were in terms of heat engines. If you wanted to apply the theory to some physical system, you first had to work out how to think of it as a kind of heat engine before you started. This was often possible, but a rather unnatural thing to do in many cases. The introduction of the concept of entropy eliminated the need to talk about heat engines and allowed the theory to be applied to virtually any macroscopic system. Further, it facilitated the discovery of statistical mechanics. The formulation in terms of entropy is formally mathematically equivalent to the earlier formulations, and thus it might be thought superfluous to requirements, but in hindsight it is abundantly clear that it was the best way of looking at things for the progress of physics.

Let’s accept my answer to the foundational question for now and examine what becomes of the earlier answers. I think it is clear that answer 2 is consistent with my proposal, and is a legitimate task for a physicist to undertake. For those who wish to take that road, I wish you the best of luck. On the other hand, answer 1 is problematic.

Earlier, I wrote a post about criteria that a good interpretation should satisfy. Now I would like to take a step back from that and urge the banishment of the word interpretation entirely. The problem with 1 is that it ring-fences the experimental predictions of quantum theory, so that the foundational debate has no impact on them at all. This is the antithesis of the approach I advocate, since on my view foundational studies are supposed to feed back into improved practice of the theory. I think that the separation of foundations and practice did serve a useful role in the historical development of quantum theory, since rapid progress required focussing attention on practical matters, and the time was not ripe for detailed foundational investigations. For one thing, experiments that probe the weirder aspects of quantum theory were not possible until the last couple of decades. It can also serve a useful role for a subsection of the philosophy community, who may wish to focus on interpretation without having to keep track of modern developments in the physics. However, the view is simply a hangover from an earlier age, and should be abandoned as quickly as possible. It is a debate that can never be resolved, since how can physicists be convinced to adopt one interpretation over another if it makes no difference at all to how they understand the phenomenology of the theory?

On the other hand, if one looks closely it is evident that many “interpretations” that are supposedly of this type are not mere interpretations at all. For example, although Bohmian Mechanics is equivalent to standard quantum theory in its predictions, it immediately suggests a generalization to a “nonequilibrium” hidden variable theory, which would make new predictions not possible within the standard theory. Similar remarks can be made about other interpretations. For example, many-worlds, despite not being a favorite of mine, does suggest that it is perfectly fine to apply standard quantum theory to the entire universe. In Copenhagen this is not possible in any straightforward way, since there is always supposed to be a “classical” world out there at some level, which the state of the quantum system is referred to. In short, the distinction between “the physics” and “the interpretation” often disappears on close inspection, so we are better off abandoning the word “interpretation” and instead viewing the project as providing alternatives frameworks for the future progress of physics.

Finally, the more observant amongst you will have noticed that I did not include “solving the measurement problem” as a possible major goal of quantum foundations, despite its frequent appearance in this context. Deconstructing the measurement problem requires it’s own special rant, so I’m saving it for a future occasion.

I have been busy reorganizing my mini-web empire, as you can see if you look at my swanky new website. Part of this has to do with the fact that I occasionally want to write about things other than the foundations of quantum mechanics, but I don’t want to burden the loyal readers of Quantum Quandaries with such trivia. Therefore, I have started two new blogs.

The first is my announcements blog. This mainly exists to serve the news feed on my website, and it will contain announcements every time I submit a paper to the arXiv, update a paper, get published, visit somewhere for a long time, unify quantum theory with general relativity etc. I won’t announce the details of every paper I write on this blog as well, unless I think the paper is interesting for people into quantum foundations (actually, on that topic you might like this recent paper and also this one). I hope you will appreciate my goal of always keeping this blog strictly on topic, bucking the trend to use blogs mainly for shameless self promotion. Of course, you are welcome to become a regular reader of my announcements blog as well, but I am under no illusions that it will appeal to anyone except maybe my mother.

Secondly, I have started another blog called Academic Tech. This should satisfy my inner geek, as it is about the uses of computers, technology and the net in academia. If you want to know about software and web tools that you can do amazing things with then you might want to read it. However, quantum theory still holds the vast majority of my attention, so articles for this blog will probably be posted much more frequently.